This invention relates to thermoplastic body solders. More particularly, the invention relates to a combination of high and low melt viscosity polyamide resins reinforced with particulate and fibrous fillers under heat and high shear to form a body solder with unexpectedly high sag resistance and excellent physical properties.
Automotive body panels are generally joined by lap welds. The resulting trough-like joints are filled in with lead or polymer based body solder and sanded to a smooth finish before painting.
Considerable efforts have been made toward developing polymeric body solders because they are lighter and less expensive than lead. However, such replacements must be as adaptable as lead for automotive assembly line use. A polymeric solder should be easy to apply and not sag on a vertically oriented surface at application temperatures or the elevated temperatures encountered in paint drying ovens. It must be compatible with automotive paint systems and should resist delamination, corrosion and cracking due to impact or thermal cycling.
U.S. Pat. No. 4,187,348, assigned to the assignee of this invention, relates to a thermoset-type polymeric body solder. It is based on the combination of epoxy, polysulfide rubber and an imidazole curing agent. The final polymerization reaction takes place after application to a car body. The principal disadvantage of the system is that once the constituents are mixed, the solder begins to cure. It must then be maintained at very low temperatures, e.g., 0.degree. C., or used within a matter of hours.
Thus it would be desirable to provide a thermoplastic rather than thermosetting body solder. A thermoplastic solder would have a virtually unlimited shelf life and could be applied to a car by remelting solid stock as done with lead solders.
It is therefore an object of this invention to provide a thermoplastic body solder which exhibits good adhesion to a metal substrate, does not sag at application or paint oven temperatures, is receptive to automotive paints, does not corrode, and is impact resistant. A more particular object of the invention is to provide a thermoplastic body solder that may be preblended and then remelted in conventional extrusion equipment for easy application to an automotive body under assembly line conditions. Another object is to provide a thermoplastic body solder resin blend having a predetermined melt viscosity that in a filled state yields a body solder with desired application properties and durability. Another object is to provide a filler system for such resin blend that suitably reinforces the body solder.
A more particular object of the invention is to provide a spreadable hot melt polymeric body solder composition based on a mixture of high and low melt viscosity polyamide resins which when blended under high shear and heat, form a stable composition having unexpectedly high resistance to sag. When such mixture of polyamides is filled with suitable powdered and fibrous fillers, it acquires and retains the desired physical properties for an automotive body solder.
A further object of the invention is to provide a method of making such a thermoplastic body solder. A more particular object is to provide a method of blending high and low melt viscosity polyamide resins to provide a body solder composition that has a high enough melt viscosity to resist sag but is easy to melt and apply to a solder joint.